Both coccolith-bearing cells (C-cells) and naked cells (N-cells) of the coccolithophorid Pleurochrysis haptonemofera can grow in salinities of more than 7° (about 20% of a "normal" sea water salinity [35°]), with the highest growth rates in salinities of more than 14?. Microscopic observations of cells suspended in 100 mM NaCl (7°) showed that, while N-cells were swelling uniformly all over the cell surface, C-cells were bulging the plasma membrane from the hole of the coccosphere at the apical (flagellar) pole of the cell. Effects of several cations and anions on the morphological change of C-cells under hypoosmotic pressure were investigated. When 100 mM K+ was used, protoplasts were released from the coccosphere completely in almost all the cells. This phenomenon was shown with K+ most effectively. The protoplasts could grow in the fresh medium and form the first coccolith within 9 h.